Intermetallic passivation of aluminum metallization

ABSTRACT

An electrically conductive intermetallic coating which is resistant to corrosion serves as a passivating layer on an aluminum metallization. The preferred intermetallic is AuA12. The intermetallic passivating layer protects the aluminum metallization from chemical attack thereby increasing device lifetime and stability.

United States Paten [151- 3,647,935

Philofsky et al. 1 Mar. 7, 1972 s41 INTERMETALLIC'PASSIVATION 0F [56] MmCited ALUMINUM METALLIZATION UNITED STATES PATENTS [72] Invent fla ymm3,436,615 4/1969 Finlayson ..317/234 ux [73] Assignee: Motorola, Inc.,Franklin Park, Ill. Primary Examin r-Darr ll L. Clay Filed Dec 15 1969Attorney-Mueller and Aichele 211 'Appl. 110.; 885,181 [571 ABSTRACT Anelectrically conductive intennetallic coating which is resistant tocorrosion serves as a pasivating layer on an alu- [52] U.S.Cl...l74/68.5,29/590, minum metalliufion. The preferred imcrmetamc is AA1,. [51] Int Cl "on 1/14 The intermetallic passivating layer protectsthe aluminum mcktheb. [58] Field ofSearch ..l74/68.5; 317/101 Q6/2524; aI y 5Claims,2l)rawing Figures l6 AuAl 2 1 INTERMETALLIC PASSIVATION FALUMINUM METALLIZATION BACKGROUND OF THE INVENTION Aluminum is the mostcommon contact metallization in use today in the fabrication of silicontransistors and in integrated circuits. Aluminum metallization is,however, subject to corrosion when exposed to halogen ions anddeteriorates rapidly as a result thereof. For example,'in plasticencapsulated discrete devices, the plastic contains fluorine, chloride,sodium, potassium and the like ions. These ions, and especially thechloride ion, under operating conditions in which water is present, tendto corrode the aluminum. The chloride ion reacts with the aluminummetallization to form aluminum chloride which dissolves in water,thereby forming a void in the strip which results in device failure.

A problem encountered with aluminum metallization in integrated circuitsoccurs as a result of the passivating glass which is placed over theintegrated circuit. This passivating glass is etched away in order tomake a contact with the aluminum bonding pad below. This passivatingglass, however, is etched unevenly with the hydrofluoric acid etchantsused, thereby resulting in corrosive attack on the aluminummetallization or bonding pads in those areas where the glass is etchedaway most rapidly. The hydrofluoric acid then etches and/or dissolvesthe aluminum metallization which results in these aluminum bonding padsbeing too thin for subsequent wire bonding.

SUMMARY OF THE INVENTION subsequently heating the gold-coated aluminummetallization to form the desired gold aluminum intermetallic, AuAl,.

Further objects and advantages of the present invention will be apparentfrom the following detailed description,.-reference being made to theaccompanying drawings wherein preferred embodiments of this inventionareshown.

IN TI-IEDRAWINGS FIG. 1 is a cross-sectional side view of asemiconductor .having an intermetallic coating on the aluminummetallization which is insulated from the substrate.

FIG. 2 is a cross-sectionalside view of a semiconductor having anintermetalliccoated aluminum metallization in contact with'thesubstrate.

DESCRIPTION OF THEI'LLUSTRATIVE EMBODIMENT As shown in'FIGS. l and 2, asiliconsubstrate l0has an aluminum metallization 14 positioned thereon.Thealuminum metallization l4may be separatedifrom the'substrate Why aninsulative layer 12, for exampleysilicon dioxide, as shown in FIG. 1 orthe aluminum metal1ization14'maybein'ohmiccontact with the siliconsubstrate l0-as shown in FIG. 2. The substrate may be any semiconductormaterial such as silicon,.germanium, and the group III-V semiconductors.Silicon is the preferred semiconductor material. The aluminummetallization layer 14 is widelyused throughoutthe industry since itmakes excellent ohmic contact with the silicon semiconductor.

In accordance with this invention, a layer of anintermetallic l6 coatsthe aluminummetallization 14. The intermeta1licl6 material has tobeelectrically conductive, that'is, have a low electrical resistivity.The electrical resistivity should be less than 20 micro-ohms-cm. Theintermetallic 16 should be resistant to corrosion, especially corrosioncaused by the chloride ion. The preferred intermetallic is AuAl, whichhas a high electrical conductivity, that is, a low electricalresistivity of about 8 microohms-cm. The AuAl, intermetallic is muchmore resistant than aluminum to attack by many acid and basic solutions.AuAl, also prevents the aluminum from electromigrating over the surface.The presence of the intermetallic AuAl, increases the lifetime ofcircuits running at high current densities and/or high temperatures.Instead of intermetallics, coatings which may be used are CrSi, NbSi,,and TaSi,. The thickness of the layer 16 is from about 1,500 to 4,500angstroms.

The intermetallic materials may be applied by standard techniques. Apreferred technique for forming a layer of AuAl, is to coat the aluminumwith a layer of gold and then heat the gold-plated aluminummetallization to a temperature to cause the gold to react with thealuminum to form the AuAh. A layer of gold having a thickness of 500 to1,500 angstroms of goldis preferred in the practice of this invention. Atemperature of from 200 to 400 C. is suitable for causing the goldcoating to react withthe aluminum metallization to form theintermetallic layer of AuAl,. The reaction time at a temperature of 350to 400 C. is of the order of a few seconds. The resultant intermetalliclayer AuAl, having a thickness of 1,500 to 4500 angstroms is coloredpurple. After the AuAl, passivating layer is formed, the wafer is thenrinsed in water and the excess gold which does not adhere to the SiO,will be washed away, thereby leaving the structure shown in FIGS. 1 and2. Gold wire is then bonded by thermal compression techniques to form astable bond therebetween.

EXAMPLE 1 A silicon semiconductor device made by the method describedabove havingan AuAl, coating 1,500 angstroms thick on the aluminummetallization was compared with a device which did not have the AuAl;passivating layer thereon for stability whensubjected to boiling waterfor 500 hours. At the end .of this time period, percent of the deviceswhich did not have the passification layer thereon failed,

whereas only 12.5 percent of the devices having an AuAl layer thereonfailed.

ln'another stability test, MIL-STD-ZOZC, method 1068, in

'whichthe V ,=-25V(65 C. for 3 hours at 98 percent RH and 25C. forliminute), 70 percent of the unpassivated devices :failed after 1,000hours, whereas 7.5 percent of the devices 102,000 angstroms.

3. -A:,method.as describedinclaim 1 wherein the step of heating saidgolddeposited, aluminum metallizationis heating to. a temperature of 200 to400 C.

4. Arsemiconductor device comprising asemiconductor substrate,

.anexposed aluminum.metallization-positioned on said substrate,.and

a layer-.of'an:electricallyconductive ,intermetallic AuAl, on .andcovering'all of. said .exposed aluminum metallization, said conductingintermetallic being relatively inert to corrosion caused by thezchlorideion.

5. A device as disclosed in claim 4 wherein an insulative layer isinterposed between said aluminum metallization and said semiconductorsubstrate.

2. A method as disclosed in claim 1 wherein the step of depoSiting saidgold is depositing gold to a thickness of 1,000 to 2,000 angstroms.
 3. Amethod as described in claim 1 wherein the step of heating said golddeposited aluminum metallization is heating to a temperature of 200* to400* C.
 4. A semiconductor device comprising a semiconductor substrate,an exposed aluminum metallization positioned on said substrate, and alayer of an electrically conductive intermetallic AuA12 on and coveringall of said exposed aluminum metallization, said conductingintermetallic being relatively inert to corrosion caused by the chlorideion.
 5. A device as disclosed in claim 4 wherein an insulative layer isinterposed between said aluminum metallization and said semiconductorsubstrate.